The present invention relates to an ignition control circuit for an engine and, more particularly, to an ignition control circuit having a failsafe function which is activated when a crank angle sensor, for detecting the rotation of the crankshaft, is broken.
In recent years, with the advent of digital electronics, digital ignition controls for engines have been developed. For example, in a motorcycle both crank angle pulses, generated from each unit angle of rotation of a crankshaft, and reference position pulses, indicating the reference position of a crankshaft, are input into an electronically controlled ignition control circuit. The ignition control circuit counts the crank angle pulses with respect to the reference position pulses and thereby sets a dwell angle and controls ignition timing.
The crank angle pulses are generated by magnetic detecting teeth formed on an outer circumference of a crank angle rotor fixed to the crankshaft. A crank angle sensor detects the teeth which are set, for example, at a pitch of two degrees. The reference position pulses are generated by a reference position sensor which detects a tooth formed on an outer circumference of a reference position rotor fixed to the crankshaft.
In the above-described ignition control circuit, if the crank angle sensor breaks, the supply of crank angle sensor pulses is discontinued and the ignition signal cannot be generated.